10w30 says it right on the oil cap.
4.5 quarts with a filter change.
Probably 4-5 quarts check after 4
I Use Chicken Oil!
Oil level sensor is on side of oil pan.
The oil filter location for the 2000 Buick LeSabre is on the front of the motor. Look just to the left of the fan belt near the left side of the motor.
On the RF of the engine underneath.
10W30 and always make sure it has the round "For gasoline engines" seal on the front.